TOMÁŠ PÁNEK, JAN HRADECKÝ, KAREL ŠILHÁN

APPLICATION OF ELECTRICAL RESISTIVITY TOMOGRAPHY (ERT)IN THE STUDY OF VARIOUS TYPES OF SLOPE DEFORMATIONS

IN ANISOTROPIC BEDROCK: CASE STUDIESFROM THE FLYSCH CARPATHIANS

Abstract. Electrical Resistivity Tomography (ERT) has become one of the most applied and user-fa-

vourable geophysical technique in geomorphological research. Multiple character of the technology

using various electrode arrays significantly reduces measurement time and is suitable for applications

even in hardly accessible mountain areas. ERT can be used for various problems concerning slope de-

formations and accompanying landform assemblages. Our study shows that the best results are ob-

tained in sites with high resistivity contrasts in subsurface environment associated with e.g. changes

of lithology or ground water conditions. Good results, which significantly extrapolate geomorphic and

speleological investigations, were obtained in elevated gravitationally spread ridges with the presence

of crevice-type caves. Resistivity soundings in such settings bring a new insight into the internal struc-

ture of deeply disrupted mountain ridges. Very promising seems to be also delimitation of bodies of

active, water-saturated landslides or lacustrine deposits behind landslide dams. On the other hand,

some problems with the interpretation of ERT record are associated with the study of internal struc-

ture and depth of old inactive landslides situated in relatively homogenous flysch layers. Although ERT

sounding results must be interpreted critically, they always shed some light on the internal character-

istics of slope deformations and in combination with other methods they create a reliable base for

the analysis of so far unrecognized features of slope deformations.

Key words: geophysics, Electrical Resistivity Tomography, geomorphology, slope deformations,

anisotropic bedrock, Flysch Carpathians

INTRODUCTION

Geoelectrical techniques belong to the most applied geophysical methods in

a broad spectrum of environmental studies (e.g. D u r a s et al. 2005; M a i l l e t et al.

2005; S o u p i o s et al. 2006, etc.). Electrical Resistivity Tomography (ERT or DC-to-

mography) has recently become the most frequently used geoelectrical application

for geomorphological purposes due to its relative simplicity and time effectivity.

This technique was used for the investigation of morphotectonics (S u z u k i et al.

2000), weathering studies (B e a u v a i s et al. 2006), fluvial geomorphology

S T U D I A G E O M O R P H O L O G I C A C A R P A T H O - B A L C A N I C A

VOL. XLII, 2008: 57–73 PL ISSN 0081-6434

L A N D F O R M E V O L U T I O N I N M O U N T A I N A R E A S

(M a i l l e t et al. 2005), permafrost detection (H a u c k and K n e i s e l 2006) or ex-

ploration of underground karst structures (Z h o u et al. 1999). ERT seems to be es-

pecially suitable for the identification of depth and internal structure of various types

of slope deformations (e.g. B a t a y n e h and A l - D i a b a t 2002; L a p e n n a et al.

2003; B i c h l e r et al. 2004; L e b o u r g et al. 2005; D r a h o r et al. 2006; P e r r o n e et

al. 2006; Sass 2006; Sass et al. 2008; Van Den Eeckhaut et al. 2007, etc.). In compari-

son with ground penetrating radar and seismic reflection Schrott and Sass (2007)

consider ERT to be the most successful geophysical method for the study of internal

characteristics of active and dormant landslides.

Slope deformations are extremely abundant features in the region of

the Flysch Carpathians (S t a r k e l 1997). However, their internal structure and

depth remain usually enigmatic features (see e.g. M a r g i e l e w s k i 2006) and

most cross-sections describing deep-seated landslides in this region are based

only on tentative, surface-related investigations (e.g. geomorphic mapping).

The aim of the presented study is to show ERT advantages and limitations when

studying the structure of various types of slope deformations and their accompa-

nying landform assemblages. The study is based on numerous ERT measure-

ments which were performed on slope deformations in mountainous regions of

the Czech and Slovak parts of the Flysch Carpathians.

METHODOLOGICAL BACKGROUND

ERT method, which belongs to geoelectric, geophysical methods, is based

on the application of electric current into analyzed bedrock and measurement of

the intensity of electric resistivity to its conduit. Basically, it gives us information on

electric resistivity properties of analyzed material towards passing electrical cur-

rent (L a z z a r i et al. 2006; S a s s et al. 2008). This data collection is realized by

means of 4 electrodes localized in a line (Fig. 1). Two end electrodes emit electric

current whose run in the bedrock has character of a part of arc of a circle.

The other two electrodes, localized between the emitted electrodes, measure the

bedrock electric resistivity in a certain point under the surface (S a s s 2006;

S c h r o t t and S a s s 2008). In this way measurement of all possible combinations

of electrodes in the profile is carried out automatically while the function of indi-

vidual electrodes changes as emitting and measuring functions alternate. This

measuring algorithm, called Schlumberger, is the most commonly used in

geomorphologic applications at the present time (S c h r o t t and S a s s 2008). Its

application is especially recommended in the research of horizontal structures

(e.g. alluvial plain), however, its use brings quality results also with regard to verti-

cal structures (e.g. tension cracks) (D r a h o r et al. 2006). Absolute maximal

depth in which we are able to measure electric resistivity is theoretically given by

maximum spacing of emitting electrodes. The absolute depth can approximately

be determined if the length of the continuously measured profile is divided by

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59

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5 to 6 (S a s s 2006; S a s s et al. 2008). The so-called multielectrode set consisting

of a random number of segments is used in terrain measurement. One segment

of this set contains eight electrodes connected by the so-called multicore-cable in

the distance of about 5.5 m. The constant distance interval between electrodes

can range from a few centimeters to a maximum value of 5 meters depending on

desired details of the research results — the smaller the distance, the more de-

tailed record (S c h r o t t and S a s s 2008). Field measurement provides us with

a field of resistivity point values in different levels below the surface. These values

are directly recorded by the measurement station and simply converted to a PC

for further processing by Res2Dinv Software (B e a u v a i s et al. 2007). The data

processing consists in the creation of the inversion of the measured data in order

to determine bedrock resistivity features (S a s s 2006; S a s s et al. 2008). Within

the software resistivity point values are interpolated and recalculated data are vi-

sualized as isolines. Right record interpretation is based on the addition of the

data on topography. Without this step, the result may be greatly deformed and

does not reflect real resistivity values of the researched rock environment. Since

the very landslide area is characterized by broken relief, the risk of wrong inter-

pretation is much higher than in the case of flat relief.

CASE STUDIES

ERT measurements realized in this study comprised typical geomorpholo-

gical situations commonly related to slope deformations in mountainous areas.

They usually involve the problematics of the depth, internal structure, degree of

activity, possible lateral continuation of failures and deep structure of rock mass

which is affected by a particular slope failure. Although more than 40 measure-

ments were made on various types of slope failures, the following chapters deal

only with the most prominent examples. All ERT measurements used Schlum-

berger electrode array with various spacing (usually 3–5 m). It enabled reaching

the depth of roughly 60 meters in the central parts of some measured sections.

If possible, obtained results were verified by other geophysical methods, bore-

holes or at least by published geological reports.

IDENTIFICATION OF DEPTH EFFICIENCY OF LATERAL SPRAEDING

ON ELEVATED MOUNTAIN RIDGES

The ridge of Čertův mlýn (1,206 m a.s.l.) is situated in the highest part of the

Moravskoslezské Beskydy Mountains. The elevated ridge is oriented in the N–S di-

rection and consists of up to several-hundred-meter thick, tectonically weakened

Godula sandstones intercalated by several-centimetre-thick layers of claystones

(Silesian unit) (M e n č í k et al. 1983). The ridge is broken by several parallel de-

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pressions (up to 100 m long and 4 m deep) indicating lateral spreading and/or top-

pling mechanism. Several entrances to shallow (up to several meters deep) crev-

ice-type caves are situated on the bottom of trenches (Wa g n e r et al. 1990).

Spreading of the ridge takes place preferentially along faults of NNW–SSE and N–S

orientations (K r e j č í et al. 2002). The whole western slope of the ridge is affected

by a large compound deep-seated landslide (750 × 600 m) where Wa g n e r et al.

(1990) described two vertical, up to ~30 m deep crevice-type caves.

ERT measurements (4 profiles) were performed across the spread ridge with

the aim to recognize the depth efficiency of gravitational spreading of the ridge axis

and verify a possibility of relation between deep crevice-type caves on the western

slope and ridge-top trenches (Fig. 2). The distribution of resistivity values correlates

well with morphological observations. Tension cracks on the ridge are associated

with nearly vertical, 25–30 m deep sectors of high resisitivity values (>2,500 ohm · m).

These high-resistivity anomalies are observable also in a section situated off the

main depression, which can serve as evidence of incipient subsurface progradation

and gravitational widening of discontinuities without morphological expressions

(Fig. 2B — see the lowermost section). The observation furthermore indicates (by

sectors with >5,000 ohm · m) possible continuation of crevice-type caves on the

ridge to deeper levels (at least 20 m) that was previously assumed (Wagner et al.

1990). The largest cross-section verifies a possibility of connection between struc-

tures situated below the ridge axis and on the western, landslide affected slope

(Fig. 2A). This is likely the result of landslide-derived unloading of the upper part of

the western slope which has caused stress relaxation and opening of tectonically

predisposed fractures above the landslide area.

VERIFICATION OF A CREVICE-TYPE CAVE

The character of this study resembles the above-mentioned investigation.

The target was the adjacent area of the Cyrilka Cave (situated in the Pustevny Sad-

dle in the Moravskoslezské Beskydy Mountains) which is the longest crevice–type

cave in the Czech part of the Flysch Carpathians (length is more than 370 m ac-

cording to Wa g n e r et al. 1990). The cave is situated just above a large trans-

lational slope deformation of dimensions 1 × 1.2 km. Unlike the previous study,

Cyrilka Cave developed in a narrow tectonic zone of the orientation of NNE–SSW,

which has, according to Klimeš and Stemberk (2007), character of a sinistral

strike-slip fault disturbing rigid, monoclinally inclined sandstones of the Godula

Formation (Silesian unit) (M e n č í k et al. 1983). The cave has character of pre-

dominantly narrow (usually up to 1 m wide) horizontal passages. Morphological

expression is connected with the presence of shallow grabens following the tec-

tonic zone, but of less conspicuous character than in the Mt. Čertův mlýn Ridge.

The aim of ERT sounding was to test whether such a narrow structure can be

identified on the resistivity record.

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ERT record clearly reveals passing of Cyrilka Cave and related fault (Fig. 3).

Vertical sector of anomalously high resistivity values (>1,000 ohm · m) is laterally

bounded by rock mass with ~55 times lower electrical resistivity. Clearly vertical

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Fig. 2. Results of ERT measurements in the ridge area of the Čertův mlýn Mt (1,206 m) in the Moravsko-

slezské Beskydy Mountains. Figure A shows the longest section (with the deepest penetration) local-

ized roughly in the central part of the disrupted ridge. Figures B are situated parallel to each other shif-

ted roughly 30 m apart. Note the lowermost profile, which is situated off the morphologically

expressive graben. In spite of this fact, high-resistivity zone in the depth of 10–20 m clearly indicates

subsurface gravitational opening of fractures and development of crevice-type caves

borders between the structures point out tectonic nature of the geophysical anom-

aly. The identified structure is more than 40 m deep and reaches below the recogni-

tion of the performed ERT sounding. The result fits previous measurement on this

site realized by a ground penetration radar (K l i m e š and S t e m b e r k 2007).

STRUCTURE AND DEPTH OF ACTIVE LANDSLIDE

Skalice landslide area is one of the largest (1.5 × 0.2 km) and most active sys-

tems of slope failures in the Czech part of the Flysch Carpathians. The landslide is

situated on the eastern, steep (mostly >20°) slope of the Skalická Strážnice Hill

(438 m a.s.l.) in the Podbeskydská pahorkatina Hilly Land (6 km SE from

Frýdek-Místek town). The landslide-affected slope is actively undercut by Mo-

rávka River, which leads to chronical unloading of unstable bedrock. The defor-

mation consists of several generations of landslides with a different degree of

activation. Last reactivation accompanied by economical losses (destruction

of several cottages) took place during extreme July 1997 rainfalls. Prior to this

event, landslide activity was recorded in 1972 (destruction of a road) and during

several phases in 19th century. Besides unloading caused by river erosion, an im-

portant precondition for the development of landslides is bedrock character. Tec-

tonically broken sandstones, limestones and shales (mapped faults are of

NNE–SSW orientation} of the Lower Cretaceous flysch of the Silesian unit are gener-ally inclined in accordance with the slope orientation.

ERT measurements were carried out in two most active parts of the landslide

area (Fig. 4). The longest section is situated in the northern part of the landslide area

where the largest landslide was activated during the 1997 floods (Fig. 4A and B).

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Fig. 3. ERT section across a tectonic zone which predisposed the development of the crevice-type

Cyrilka Cave (Pustevny Saddle, Moravskosleyské Beskydy Mountains). Note that the profile is situated

roughly 50 m off the entrance to the cave; despite this shift, its structure is very clearly visible in the re-

sistivity record

Several rotated blocks with fresh tension cracks and minor headscarps are delim-

ited at the upper part of the failure with rectilinear 15-m high headscarp. Margin of

a flat ridge above the headscarp is disrupted by several active tension cracks (ac-

companied by crevice-type caves), up to 3-m-deep trenches and related sinkholes.

High-resolution ERT measurement (3m spacing of electrodes) in the zone of ten-

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Fig. 4. ERT profiles in the Skalice landslide area (Podbeskydská pahorkatina Hilly Land). Figures A

and B display the structure of the largest active landslide in this locality, whereas the uppermost sec-

tion shows high-resolution ERT record of the tension zone just above the main headscarp. Figure B

shows internal structure of a small July 1997 rotational landslide

sion cracks reveals up to 10-m-deep high-resistivity sectors indicating subsurface

progradation of cracks and possible presence of caverns. ERT profile with 5m spac-

ing of electrodes covers the whole slope failure including both the area above the

main headscarp and the landslide body itself. Vertical electrical contrasts in the

zone of tension cracks and main headscarp verify morphological observations and

point out tectonic predisposition of the landslide. Rock mass below tension cracks

is affected by curved resistivity boundary, which can be evidence of advancing

shear surface and possible future position of the headscarp (Fig. 4B). Landslide ac-

cumulations below the main headscarp are interpreted as low-resistivity (perhaps

water-saturated) bodies of the depth of 10–20 m. Shear surface follows probably the

contact between thinly-bedded Těšín-Hradiště Formation overlying thick layers of

Těšín limestone (high resistivity zones in the middle part of the section).

Other ERT measurements were performed on a small July1997 rotational land-

slide in the southern part of the area (Fig. 4C). Although resistivity record is not fully

unambiguous (and is perhaps affected by dry conditions during the measurement),

the rotated block frontal part at the foot of the slope and several possibly minor shear

surfaces inside the slope deformation can be recognized on electrical contrasts.

ERT sounding performed in the area of the Skalice landslide area reveals

deep, structurally preconditioned nature of individual slope deformations. In con-

trast to results obtained in the area by inclinometric measurements (Kovář, L. per.

com, 2006) which detected only most shallow shear surfaces, individual land-

slides in the studied site seem to be segmented into several levels in various

depths and are likely deeply rooted.

INTERNAL STRUCTURE AND DEPTH OF A LARGE DEEP-SEATED COMPOUND

LANDSLIDE

One of the largest landslides in the Czech part of the Flysch Carpathians is

a mostly inactive compound failure situated on the western slopes of Velký Stožek

Mt. (978 m a.s.l.) in the Slezské Beskydy Mountains. The slope deformation includes

the area of ~2.5 × 0.9 km and affects monoclinally inclined Godula and Istebna

Formations of the Silesian unit (M e n č í k et al. 1983). Published data from a bore-

hole situated in the vicinity of the landslide indicate a more than 100-m-thick com-

plex of rather homogenous sandstone layers with only thin intercalations of shales.

A majority of the landslide is developed on a slope conforming to bedding planes.

The landslide is mostly inactive, but has conspicuous morphology with several gen-

erations of headscarps, rotated blocks and undulating topography.

One of the longest ERT profiles (435 m) covering the whole length of the slope

from water divide to the adjacent valley bottom was realized in the studied site

(Fig. 5). Resistivity record seems to be rather complicated, showing block nature of

the slope failure, probably multiple shear surfaces and depth of the whole slope

failure ~50–60 m. The landslide probably consists of two levels of failures, the up-

65

per one of which is characterized by a low-resistivity sector (<200 ohm · m) reach-

ing the depth of 15–20 m. However, the internal structure and resulting ERT record

seem to be too complicated for a straightforward conclusion and should be verified

by other geophysical methods.

DEPTH OF A RECENT FLOW-LIKE LANDSLIDE

Hluboče landslide (the failure developed between 3 and 4 April, 2006) is the

largest landslide which has occurred in the territory of the Czech Republic in the

last decade. The site is situated on substratum formed by claystone-dominated

flysch of the Magura unit in the Bílé Karpaty Mountains. The flow-like landslide

developed as a consequence of over-saturation of weathered bedrock due to in-

tensive rainfalls and rapid snowmelt at the turn of March and April, 2006. Al-

though situated in a mountainous area, it caused the destruction of several

houses, forest road and pastures. The landslide of the length of ~770 m and maxi-

mum width of 110 m has two headscarps which overlap in the central part of the

failure and under of this junction continues as a earhflow. Material situated in

both depletion zones has not been evacuated yet and thus creates unstable part

of the slope for potential future landslide activization. ERT measurements on this

slope failure were part of more complex landslide research which was realized

after the catastrophic event (K l i m e š et al., in press).

The aim of ERT sounding was to infer the thickness of landslide-affected rock

mass in both depletion zones of the flowslide (Fig. 6). Three profiles describing

66

Fig. 5. ERT longitudinal profile of the Velký Stožek Mt. (978 m) compound landslide

longitudinal and cross-sectional characteristics of the landslide were applied. Re-

sistivity record clearly shows the thickness and geometry of water-saturated land-

slide material. Low-resistivity layers (<80 ohm · m) indicate the depth of “remain-

ing” material (up to 10–15 m) in both depletion zones (Fig. 6B and C). Cross-sec-

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Fig. 6. Structure of depletion zones of an April 2006 Hluboče landslide (Bílé Karpaty Mountains).

A — detail of a cross-section situated in the western headscarp of the flow-like landslide, B — Longitu-

dinal section of the upper segment of the western headscarp, C — Longitudinal section of the upper

segment of the eastern headscarp

tion through the western headscarp area furthermore indicates weathered, tec-

tonically weakened bedrock (Fig. 6A) that is indicated by almost vertical low-re-

sistivity zone (<40 ohm · m). The presence of such a fault-related weakened zone

(with numerous springs) was expected as a main precondition factor already

during geomorphic mapping prior ERT measurements.

DEPTH OF LACUSTRINE DEPOSITION IN AN ANCIENT

LANDSLIDE-DAMMED LAKE

Frequently solved problems in geomorphology of slope deformations con-

cern various types of landslide-related deposition such as lacustrine/swamp de-

posits in near-scarp depressions or impoundments behind landslide dams. Our

case study deals with one of the most conspicuous fossil landslide-dammed lake

(area of the contemporary dry impoundment is ~7 ha) situated in the Slovakian

part of the Flysch Carpathians (Kysucké Beskydy Mountains in the vicinity of

Oščadnica village). The lake developed as a consequence of a huge landslide

which affected the whole area of the southern slopes of Kykula Mt. (1,087 m) and

is nowadays reflected also in a ~50-m-high step in the longitudinal profile of the

valley. The aim of ERT measurements was to provide information for further drill-

ing works and infer information about the depth of lacustrine sequences and their

relation both to the bedrock topography and the landslide body.

Three ERT profiles were localized across the former lake bottom and adja-

cent slopes (Fig. 7). All sections demonstratively depict topography of the buried

valley bottom. Low-resistivity (<40 ohm · m) lacustrine sequences clearly con-

trast with the landslide material and bedrock. ERT indicates the depth of lacus-

trine deposits to ~10–19 m in the deepest parts of the cross sections. Further-

more, a section situated close to the landslide dam nicely shows geometry of the

landslide toe which caused the valley blockage (Fig. 7C). High-resistivity landslide

material consisting of large sandstone boulders is delimited in accordance to

bedrock slope by low-resistivity (<50 ohm · m) shear zone (see Fig. 7C).

DISCUSSION OF ADVANTAGES AND LIMITATIONS OF ERT

IN LANDSLIDE STUDIES

The study of inner structure is one of key tasks in the research of slope defor-

mations. However, natural outcrops that provide researchers with this type of in-

formation are rarely available in geomorphologically oriented research. There-

fore, other methods, both destructive (e.g. excavation pits or drilling) and non-de-

structive methods including a wide range of geophysical methods, must neces-

sarily be applied (for details see D u r a s et al. 2005; S a a s et al. 2008; S c h r o t t

and S a a s 2008). Research of slope deformations in anisotropic flysch of

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the Western Carpathians represents suitable environment for the application of

geophysical methods, particularly ERT, as research based on natural outcrops or

drilling is practically impossible or largely limited in mountain conditions.

Important issues in the inner structure study include the reach depth of

a given method and details of partial structures of a slope deformation. This fact is

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Fig. 7. Cross-sections localized at the bottom of a ancient landslide-dammed lake (Kysucké Beskydy Mountains).Figures A and B show internal structure and topography of a buried valley in the proximal and middle parts of thelake respectively. Figure C reveals the structure of the distal (near-dam) part of the landslide-dammed lake. Note

the cross-section right part which clearly displays landslide toe overthrust onto the opposite slope

particularly relevant with regard to the recording of potential slip surfaces. In this

respect, precise identification of the main slip surface or partial slip surfaces is

very problematic and it generally requires using other methods, non-geophysical

methods included (compare S c h r o t t and S a s s 2008).

Well identifiable boundary lines are results of high resistivity contrasts

( S c h r o t t and S a a s 2008). However, this effect may have various causes that

might not necessarily be evident in the interpretation of tomographic record. Ap-

plication of more methods is therefore desirable (e.g. Pe r r o n e et al. 2004).

The boundary line may not be well identifiable also because intervals of measur-

able resistivity of various types of rock environment show considerable overlaps

( S a a s et al. 2008). For example Ch. K n e i s e l (2003) mentions clay resistivity

values of 1–100 ohm · m and groundwater resistivity values of 10–300 ohm · m.

Both the physical environments are typical identifiers of slip surface (S a a s et al.

2008). Slip zone can show very low resistivity, however, at the same time it can be

a clay layer or groundwater zone that did not necessarily function as slip surface.

This fact can play a very important role in precise localization of slip surfaces in

the very flysch conditions. Good contrasts show a boundary line between weath-

ered cover and bedrock (B e a u v a i s et al. 2007; S c h r o t t and S a a s 2008),

which is a zone of rock environment instability and landslide body detachment in

shallow landslides. Determination of the main slip surface is also limited by maxi-

mum obtainable depth by ERT method.

Often, only partial slip surfaces can thus be recorded. O. S a a s et al. (2008)

claims individual landslides blocks can be well identified and distinguished from

bedrock structures in silty, comparatively wet and conductive substratum. Less

contrastive environment makes drilling and inclinometric measurements the only

reliable methods of the verification of geophysical measurement, on the other

hand, also very costly. In the context of the case study of active Skalice landslide,

ERT method application was considered to be beneficial as it revealed a much

deeper slip surface of the slope instability than it was recorded by inclinometric

measurement.

ERT method is limited by the presence of coarser regolith (blocky surface

or large debris cover) on the surface, which does not allow optimal introducing

of electric current into the bedrock. As a consequence, the resulting record can

show wrong resistivity distributions that are not primarily displays of resisistivity

conditions inside the rock body but technical measurement errors. This effect

can appear in conditions of block fields that can accompany certain types of

slope deformations (e.g. rock avalanches). In this case, the application of ERT

method can be practically excluded. However, the method can be applied in

heavily disrupted rock environment containing vertical or subvertical caverns

or crevice-type caves. However, a good contact of surface layers with electro-

des must be ensured. The occurrence of non-homogeneities in the rock body

may not be evident in the georelief character and thus the very ERT method can

very well identify the presence of these high resistivity structures.

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Relatively easy application of the method in terrain and possibility of re-

peated measurement seem to be great advantage in the monitoring of active

slope deformations. This fact is supported by a case of Hluboče flow-like land-

slide where increased water saturation of rock environment represented a critical

triggering mechanism. ERT method is very effective in the monitoring of active

landslides and determination of stable conditions (see also S a a s et al. 2008).

Repeated measurements and subsequent result comparison need to be ac-

companied by the monitoring of measurement conditions (esp. climatic conditions)

which can distort resulting interpretations of slope predisposition to landsliding (e.g.

F r i e d e l et al. 2006). The location of the boundary line of landsliding body and

“healthy” bedrock represents very important information also for possible landslide

mitigation or stabilization of the displays of active landsliding. The usefulness of ERT

method for the monitoring of active landslides is thereby more than obvious.

CONCLUSION

ERT geophysical method represents a suitable tool in the research of various

aspects of slope deformations in flysch structures, however, particularities of this

type of geological structure need be taken into account. Important aspects of the

application of ERT method in geomorphologic research of slope deformations

are electrode configuration and geoelectric sounding depth. Case studies show

convenience of using Schlumberger electrode array with different electrode

spacing and length of profile to obtain results with required detail and depth. ERT

method represents an effective tool in the research of fossil, deep-seated slope in-

stabilities to which the occurrence of pseudokarst caves is related.

ERT method can also be used in the research of active slope deformations, in

order to not only localize inner structure elements, but also monitor them. In terms

of the chronology of slope deformations and role of landslides in erosion denuda-

tion processes in a basin, ERT method can be used in the study of sedimentary ar-

eas of various positions towards the respective landslide. The above mentioned

suggests that ERT method is a convenient method that on the one hand brings origi-

nal results, but on the other hand, needs to be verified by other methods.

In the ERT method application, right interpretation of results requires relating

the results to other, mainly geomorphologic and geologic, information.

ACKNOWLEDGEMENTS

This research was supported by a grant from the Grant Agency of the Czech

Republic, No. 205/06/P185: “Comparison of morphotectonics and geomorphic

effect of surface uplift in the highest part of flysch Western Carpathians and

Crimean Mountains”.

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Department of Physical Geography and Geoecology

Faculty of Science, University of Ostrava

Chittussiho 10, 710 00 Ostrava, Czech Republic

tomas.panek@osu.cz

jan.hradecky@osu.cz

karel.silhan@osu.cz

STRESZCZENIE

T o m á š P á n e k , J a n H r a d e c k ý , K a r e l Š i l h á n

ZASTOSOWANIE ELEKTROOPOROWEJ TOMOGRAFII (ERT) W BADANIACH DEFORMACJI

STOKÓW W PODŁOŻU ANIZOTROPOWYM: STUDIA W KARPATACH FLISZOWYCH

Zastosowanie badań elektrooporowych (ERT) stało się powszechną techniką w badaniach geomor-

fologicznych. Złożony charakter technologii, w których są używane różne zestawy elektrod, znacznie

skraca czas pomiarów terenowych i są użyteczne w trudno dostępnych obszarach górskich. Są uży-

wane do rozwiązywania różnych problemów, a najlepsze wyniki uzyskuje się przy badaniach

deformacji stoków górskich i zespołów form tworzonych przez te procesy.

Artykuł przedstawia najciekawsze wyniki uzyskane na obszarach, gdzie występują największe

kontrasty oporności elektrycznej w podłożu, związane ze zróżnicowaniem litologicznym lub wodami

gruntowymi. Dobre rezultaty uzyskano zwlaszcza w badaniach speleologicznych w obrębie elewo-

wanych grzbietów z jaskiniami szczelinowymi. Sondowania elektrooporowe pozwalają rozpoznać

wewnętrzną strukturę grzbietów górskich.

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Bardzo obiecujące wydają się również studia zmierzające do określenia zasięgu stref mobil-

nych w aktywnych, nasyconych wodą osuwiskach lub w osadach jeziornych, po wewntęrznej stronie

zapór osuwiskowych. Trudności powstają przy interpretacji zapisów ERT w starych, nieaktywnych

osuwiskach, zwłaszcza w obrębie względnie homogenicznych warstw fliszowych. Pomimo potrzeby

krytycznego spojrzenia na sondowania ERT, rzucają one jednak trochę światła na wewnętrzną struk-

turę deformacji stokowych, a przy równoczesnym użyciu innych technik geofizycznych, tworzą dobrą

podstawę do badania skutków ruchów masowych na stokach górskich.

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